In recent years there has been increasing interest in the manufacture of composite materials utilizing ceramic whiskers, fibers or platelets as the material for reinforcing metallic, polymeric or ceramic matrix materials. This not only strengthens most metallic and polymeric matrix materials, but also provides an attractive toughening mechanism for ceramics. For example, platelets substantially increase the hardness and abrasion resistance of aluminum alloys. Important criteria used for choosing the optimum material in a particular application are the ratio of strength to weight, and the ratio of modulus of elasticity to weight.
Silicon carbide in the form of whiskers and platelets have been the primary materials now under consideration for such use in composite materials. Silicon carbide is used as it has excellent mechanical properties, good thermal conductivity, good oxidation resistance and is a semi-conductor with superior properties. In most applications silicon carbide in the form of whiskers (single crystal discontinuous fibers) have been used. However, there are some problems encountered with the use of whiskers:
(a) it is difficult to obtain a uniform dispersion in the matrix materials because the whiskers tend to agglomerate and are difficult to disperse--they also have a tendency to reagglomerte. PA1 (b) manufacturing costs, and thus selling prices, are high. PA1 (c) whisker materials are relatively impure which causes compatibility problems with the matrix material being reinforced.
In contrast, the use of silicon carbide platelets solves many of the problems inherent with the use of silicon carbide whiskers. The platelets can be made to be very free flowing so that mixing of the platelets into a host matrix is relatively easy. Also, higher "loadings" with the platelets can be achieved and the platelets exhibit enhanced wetting when incorporated into metallic materials. Silicon carbide platelets can be made to a very high level of purity: almost 1000 times more pure than silicon carbide whiskers. The main problem, however, is the cost of their manufacture.
The term "platelets", as used herein, is to be understood to apply to crystalline structures having an average thickness from about 0.5 microns to about 150 microns, and a diameter from about 3 to about 100 times larger than the thickness. The smaller diameter of these materials are sometimes referred to as "flakes" which often are obtained as a by-product of the production of silicon carbide powders, for example. It is these flakes that are most commonly used. The platelet crystal can consist of metals, metal carbides, metal borides, metal nitrides, metal oxides, etc.
As stated above, one of the most widely used and researched of the whisker and platelet materials is silicon carbide. Because of its importance, numerous processes have been developed for the production of these silicon carbide materials. For the production of silicon platelets, for example, a process is described in U.S. Pat. No. 3,520,740 issued to A. Addamiano on July 14, 1970. In this patent the product is produced by reacting a mixture of SiH.sub.4, C.sub.3 H.sub.8, and H.sub.2 in a bell jar. In another patented process described in U.S. Pat. No. 3,011,877 issued to H. Schwieckert, et al., hydrogen is reacted with silicon tetrachloride or silica-chloroform on a heated strip, such as tantalum. These are but a sample of the many patents related to the production of silicon carbide platelets.
Other patents in the general technology are: U.S. Pat. No. 4,387,080 issued to Hatta, et al., on June 7, 1983; and U.S. Pat. Nos. 3,174,827 and 3,230,053 issued to N. T. Wakelyn, et al.
One of the problems encountered with the processes of the patents for production of silicon carbide platelets and whiskers is the low yield. Most of the described processes require plating onto some type of substrate material which involves subsequent removal. This removal step is slow, tedious, and thus expensive. Another problem is that these processes are low volume production processes. Also, some of the processes require expensive and hazardous starting materials. Furthermore, several of the processes produce a product that only slightly resembles a platelet: the opposite faces are only generally parallel.
Accordingly, it is a primary object of the present invention to develop an economical process for the preparation of silicon carbide platelets.
It is another object of the invention to develop a process for the preparation of silicon carbide platelets that has high yield and can be performed in production-sized apparatus.
It is also an object of the present invention to provide a process that does not require a substrate for the formation of the product, and a process that does not use hazardous materials.
It is a further object to provide a process for the preparation of silicon carbide platelets having substantially parallel and flat opposite surfaces.
A further object of the present invention is to provide a process whereby silicon carbide platelets can be produced in selected ranges of sizes.
Still another object is to provide a process whereby the platelets are singular, as contrasted to complex platelets, so as to eliminate loss of material that occurs when complex platelets are mechanically converted to single platelets.
These and other objects of the present invention will become apparent upon a consideration of the full description of the invention given hereinafter.